Optical testing device and method of testing



De@ 15, 1942 c. J, GLAssER OPTICAL TESTING DEVICE AND METHOD OF TESTING 3 Sheets-Sheet l Filed- March 2 0, 1941 lillllala'lilililililimilf r INVENTOR l CQr/eSJ/Qef Dec. l5, 1942. C, J GLASSER I 2,304,814

OPTICAL TESTING DEVICE AND METHOD OF TESTING Filed March 20, 1941 3 Sheets-Sheet 2 I Lililil:lilililiialilili| fra/6:72 fr' ,Ugg

Dec. l5, 1942. C 1 GLASSER 2,304,814

OPTICAL TESTING DEVICE AND METHOD OF TESTING Filed March 20, 1941 3 Sheets-Sheet 3 Patented Dec. 1.5, 1942 UNITED STATES PATENT OFFICE OPTICAL TESTING DEVICE AND METHOD F TESTING Charles J. Glasser, Chicago, lll.

Application March 2o, 1941, serai No. 384,3:2

25 claims. (ci. asf-56) This invention relates to a method of and means for testing lenses to determine their focal characteristics, and is applicable to individual lenses or combinations of lenses which may be placed in the test-apparatus, as well as to such lenses as cannot be placed in the test apparatus, for instance, the lens of the human eye.

In the testing of lenses as, for instance, to determine the focal length of a lens, in accordance with the principles of one embodiment of the present invention, light is directed to the lens under test, and then passed to two photo-electric cells so arranged that one cell receives all of the light on one side of the optical axis of the lens and the other cell receives all of the light on the other side of the optical axis of the lens. The current or potential of those cells is sent or applied to a. balancing circuit, the output of which is a function of the difference between the outputs of the two `cells (suitable Aamplifying means being provided). A light cut-oil is provided for cutting oil the light entering one ci' the cells, without substantially affecting the light entering the other cell. This cut-oil is shifted longitudinally along the optical axis of the lens. So long as the cut-off does not pass the point where the light rays cross the optical axis of the lens the movement of the cut-ofi produces no relative change in the two light cells. One light cell remains entirely cut oil and the other light cell is not at all out off, and therefore the outputs of the cells remain unbalanced. When, however, the cut-olf reaches the cross-over point where the light rays cross the optical axis of the lens the cut-oil intercepts the light from neither cell, whereupon a balanced condition of the cells is reestablished. This balanced condition is broken immediately as the cut-off passes the cross-over point. Thisgives a sharp indication of the crossover point of the lens, from which the focal length is determined.

Since the light cut-off from one or the other of the two photo cells is zero at the cross-over point and rises to 100% value for one or the other of the cells as the cut-off member moves only a very slight distance from the cross-over point, there isthus obtained a sharpl cut-oil! point indicating when the cross-over point-of light from the lens is reached. This sharp cut-off point may be utilized to actuate a relay to mark a graph sheet that moves together with the cut-oil and thus indicate on the graph sheet the exact point of cross-over. Thus, if a series of lenses are to be tested the tests may be performed by an unskilled operator and the necessary conclusions, for

which skill is needed, may be drawn by another person from an examination of the charts thus made. Furthermore, in filling a prescription for eye glasses the lens manufacturer can furnish a chart of the test made upon the glasses furnished to the patient, and the doctor can readily ascertain whether or not the glasses furnished the patient are in accordance with the prescription.

While in the preferred embodiment of the present invention it is desirable to have two light cells arranged in opposition, it is within the purview ofthe present invention to use only one light cell. 'Ihe cell receives approximately half of the light when the cut-ofi is on one side of the cross-over point of light from the lens, regardless of the precise location of the. cut-oil, and receives zero light when the cut-oil is on the opposite side of the cross-over point. Thus the location of the cross-over point is sharply apparent as the electric output of the cell goes from zero to its full value.

It is a further object of the present invention to provide an apparatus and means for testing lenses which is applicable to spherical lenses and also applicable to astigmatic lenses, and which can be used to ascertain the axes of astigmatism and to ascertain the focal lengths on the respective axes.

It is a still further object of the present invention to provide a lens testing apparatus the functioning of which will be independent ofthe de-l gree of luminosity of the light source used. As a result it is possible to use an ordinary electric light source energized from a commercial power system which may be subject to iluctuations and/or surges. The arrangement is suchl that light from the light source actuates two light responsive cells connected differentially. If the luminosity of the light source increases or decreases it will aiect both light cells equally. Hence the system will not be influenced by variations in `luminosity of the source, whether the variations are due to changes in the light bulb itself or due to electrical surges.

The attainment of the above and further objects of the present invention will be apparent from the following specification taken in conjunction with the accompanying drawings forming a part thereof.

In the drawings:

Figure 1 is a diagrammatic View of an optical system embodying the present invention;

Figure 2 is a horizontal sectional view, parl tially diagrammatic, of a structure operating on the principles of Figure l:

. Figure 3 is a fragmentary sectional view taken along the line 8-3 oi' Figure 2;

Figure 4 is an end view of Figure 3;

Figure 5 is a diagrammatic view illustrating another embodiment of the present invention; Figure 8 is a diagrammatic horizontal sectional view Vof a structure operating on the principles illustrated in Figure 5;

Figure 7 is a diagrammatic view illustrating stili another embodiment of the present invention: l

Figure 8 is a horizontal sectional View through a testing apparatus for testing the lens of the human eye; and

Figure 9 is a diagrammaticyiew illustrating another embodiment of the present invention.

Reference may now be had more particularly to Figure l. In this figure a light source, indicated at I, directs light through a narrow slit or aperture 2 in a plate 2 to a lens 4 which is to be tested. The lens is mounted at a fixed distance from the slit 2 and directs its light to a reflecting prism 8, one surface 'I of which reflects light to an electric light responsive cell 9 and the other surface 9 of which reflects light to an electric light responsive cell I 0. A masking screen or cut-ofi I2 is provided between the reflecting prism 6 and the test lens 4. j The cut-off has a knife edge at its lower end which terminates exactly on the optical axis I4 of the test lens 4 or an exceedingly minute distance above the axis I4. The slit 2 and the apex between the prism surfaces 'l and 9 are also on the optical axis I4. The mask I2 is-movable between the prism B and the lens 4 while maintaining its lower knife edge always on the optical axis I4. When the cut-off is in the position illustrated in full lines in Figure 1 it intercepts substantially all of the light from the lens which would otherwise strike the prism surface 1 but does not affect the light from the lens which strikes the prism surface 9. This condition prevails unaltered so long as the cut-off I2 moves between the prism 8 and the point I 5which is the cross over point where light rays from the lens 4 cross the optical axis I4. At the cross over point the knife edge of the cut-off I2 permitslight to pass to both prism surfaces 1 and 9. When the cut-E I2 moves to the left of the cross-over point I it cuts oil' all the light from the lens 4 that` would otherwise strike the prism surface 9 and cuts off none of the light from the lens 4 that strikes the surface 1. Thus, for every position of the cut-'off I2, other than the position at I5, the cut-off masks out all of the light from only one of the light cells, and at the position I5 it masks i out the light to neither of the light cells. The position I5, it is apparent, is determined by the focal length' of the lens 4.

The outputs of the light cells 8 and III are delivered to a bridge or differential typeamplifler Il. When the two cells receive equal amounts of light the electrical effects of the two cells in the amplifier neutralizes one another.

the cut-off momentarily crosses through the cross-over point I5. At this instant the balance is reestablished and the amplifier output again The cut-off is then moved drops to zero. If the cut-ofi moves past. the cross-over point Il the balance is immediately again destroyed. The establishment of balance indicates that the cut-oi! lI2 is at the cross-over point I5 of the lens 4. A suitable scale Il is provided, which is appropriatelycalibrated, from which the position of the cut-off can be read. thus reading the focal length of the lens 4 directly. The output of the amplifier goes to an electric device I6. which maybe a visual or audible signal device, or may be a relay for controlling a recordk ing device, or the like.

If the lens 4 under test is a spherical lensxthe image of the line slit 2 at the focal point Il will be a point. Hence the angular position of the cut-off I2 at the point I9 will be immaterial, and the cut-off I2 together with the prism may be rotated about the' axis I4 as a center with'out affecting the balanced condition of the light cells. Also, the cut-on may be rotated independently of the prism without affecting the balanced condition of the'light cells. If the lens is, however,` astigmatic, then the image of the source of light 2 at the pointr I5 will be a line, rather than a point, and if the knife'edge is rotated with respect to the image, while leaving the prism stationary, the balance will be disturbed. This condition may therefore be utilized to determine whether or not the lens is astigmatic, for if the cut-of! is rotated at the focal point and the balanced condition isnot disturbed then the lens is not astigmatic.

In the description thus far Vgiven it=was as-x sumed that the lens under test vwas a spherical lens or, if there is astigmatism, it was assumed that the axis of astigmatism was horizontal, that is, parallel to the knife edge ofthe cut-off I2. If, however, the axis of astigmatism of the lens 4 happens to be inclined at an unknown angle with respect to the knife edge of the cut-off, then movement of the cut-off along the optical axis of the lens will not produce a change from the condition of balance to the condition of unbalance of the system, or vice versa. 'Ihis indi'- cates that there is astigmatism in the lens and that the axis of astigmatism is atan angle to the knife edge. It is now necessary to ascertain the axis of astigmatism. This result is obtained in vthe following manner:A The cut-off I2 and the prism Ii are rotated with respect to one another about the optical axis I4 as a center until the knife edge of the cut-oil* is at right angles to the edge of the prism at the intersection ofthe surfaces 1 and 9. If there isaStigmatism inthe lens under test a cross section of thef beam of light from the lens will be an ellipse whose major and minor axes are at right angles to` one another and at an inclination to the horizontal determined by the meridian of astigmatismof the lens. The cut-off I2 intercepts half of the section of the beam. The edge of the prism 6 being at right angles to the edge of the cut-off therefore cuts the remaining half of the beam of light into two quadrants. `If the edge of the cut-off I2 happens to be parallel to an axis of the ellipse then the two quadrants into which the non-intercepted half Aof the beamv is divided by the prism will be of equal areas, and the light effects on the two surfaces 1 and l of the lens will bethe same. If, however, the knife edge I2 cuts the ellipse of light along a line other than one of the axes of the ellipse then the division of the ynon-intercepted half of the beam by the 2,304,814 knife 'lz and the prism s together about the am` I4 as a center, thev relative areas of these two quadrants is changed. 'I'he areas remain unequal until the knife edge comes into a position parallel with one of the axes of the ellipse of light while they are maintained at right angles to one another, there is a, change in the condition of balance between the light cells as the knife edge passes through a position parallel to an axis of astigmatism. One axis of astigmatism vof the lens is thus ascertained to be parallel to the knife edge. The prism is then rotated to bring its apex into parallelism with thel knife edge of the cutofi.' (or the cut-01T is rotated to bring it into parallelism with the prism edge). The knife edge may then be moved along the optical axis I4, without rotating the knife edge, until it reaches the focal point of the lens at that axis. In that position and only in that position is balance reestablished. This thus indicates the meridian of one axis of astigmatism and the focal length in that axis. The cut-off and the prism are then rotated through exactly 90 and the cut-off I2 is then again shifted along the optical axis I4 to ascertain the focus of the lens in the op-A posite meridian, at right angles to the first meridian. Thus the focal length and angle of astigmatism of a lens can be found.

Reference may now be had moreparticularly to Figures 2, 3 and 4, illustrating, diagrammatically, a physical structure for carrying outy the principles of the system of Figure 1. The structure includes a base having two upwardly extending parallel inverted V-shaped tracks 2I-2I upon which a holder 22 for the cut-off I2 is provided. The holder 22 is slidable along the tracks 2I--2I. It has a circular opening in which is rotatably mounted a, ring 24 that carries the cutoff I2. The cut-oil is in the form of a semi-circular disc having a knife edge at the bottom.

The holder 22 embraces a lead screw 25 which threads through the holder and is rotatably supported in suitable bearings, not shown. Upon rotation of the lead screw 25 the holder 22 is moved in one direction or the other along the tracks 2l. The lead screw 25 is rotated by a worm wheel 26 keyed thereto in mesh with a worm on a shaft 28. The shaft 28 is turned b hand, as by a knurled wheel 29.

The llens 4 to be tested is mounted in a holder 30 which may be of any desired construction and malaises toy both cells, and equilibrium is estab- The various parts .are rotated by a rotatable shaft 48 that is turned. by hand and carries worms 4I in mesh with worm wheels 42-43 respectively. The worm wheel 42 drives a shaft 44 to which are splined a clutch 45 and a clutch 46.

The clutch 45 is actuated by a crank on a stud vshaft 41, and the clutch 46 is actuated by a crank on a stud shaft 48. The clutch 45 is adapted to engage or disengage the shaft 44 with a pinion shaft 58 which is in mesh with a gear 5I on the cut-olf carrying ring 24. Ihe pinion Il extends the full length of the path of longitudinal travel of the cut-oil.' I2, so that the gear 5I is in mesh with the' pinion SII-throughout the length of the longitudinal travel of the cut-off. Upon rota-` tion of the pinion 50 the gear 5I is rotated to rotate the cut-off I2 about the optical axis I4.

The clutch 46 is adapted to engage or disengage the connection between the shaft 44 and apinion shaft 54 which is in mesh with a spur gear 55 that rotates the slit plate '3. The slit plate 3 is longitudinally -slidable for reasons which will be more fully explained hereafter, and for that reason the pinion 54 is elongated.

The worm wheel 43 drives a shaft 58 which, through a clutch 58, actuated by a stud shaft 60, establishes driving connections with a shaft 6I carrying a spur gear 62 in mesh with another spur gear 63 that rotates the casing 35 to rotate the prism 6 and associated light cells 8 and I0 in the same direction and at the same speed as the direction and speed of rotation of the cutoff I2. Thus by controlling the clutches and turning the shaft 48 it is possible to rotate the slit 2 or the cut-off I2 or the prism 6, or any combination of the three. The clutch shafts 41,-48 and are manually actuated.

Means is provided for reading the angular and the linear positions of the cut-oil` I2. To accomplish this the pinion shaft 58 carries a worm 65 that drives a worm wheel 66 which turns a shaft 61. The shaft 61 has a spiral miter gear therewhich is stationary -on the base 28. The plate 3 which has the slit 2, is rotatably mounted in a sleeve 32 that is slidable on the tracks 2l of the point, the cut-oil does not disturb the light which on-in mesh with a spiral miter gear 68 which rotates a shaft 69 that carries a miter gear 'I8 in mesh with a miter gear 1I on avshaft 12 which carries a calibrated disc 13. 'I'hus the disc 13 is rotated together with the cut-off I2. The cutoff I2 has a maximum rotation of 180. The gear ratio is such that the disc 13 rotates twice as fast as does the cut-off I2.

To ascertain the linear position of the cut-off I2 the shaft 28 isprovided with a pair of spiral miter gears 15-16 (Fig. 3) of different sizes, loose on the shaft and arranged so that either one. but not both, may be keyed to the shaft by a clutch 11. The miter gear 15 drives a gear 18. and the miter gear 16 drives a gear 'I9both of which are in mesh with a gear 88 that drives a shaft 8I. 'Ihe shaft 8| through a worm 82 and worm wheel 83 drives a shaft 84. The shaft 84 carries a spur gear 85 in mesh with a gear 81 on a shaft 88. The shaft 88 rotates at a speed one-tenth that of the shaft 84. The shafts 84 and 88 have calibrated discs 90 and 8| thereon. 'I'he arrangement is such that rthe disc 9| makes one complete revolution when the lead screw 25 has been turned a sufficient number of times to move the cut-off I2 the full length of its rectilinear travel. During that time the disc 98 makes ten revolutions. The disc 9I may be calibrated in inches and tenths of an inch. The disc is similarly calibrated so that each inch calibration corresponds to a tenth of an inch of travel be in diopters or any other scale.

of the holder 22. If desired the calibration may A greater number of multiples may be obtained by a greater number of gears orratios thereof.

The ring 24 is constructed toreceive the cutof! plate I2 in either of two positions lat right angles to one another. In one position of the cut-off plate I2 the knife edge thereof is parallel with the apex-of the prism 0. In its alternate by a circuit controlled from `the amplifier I8.`

The arrangement is such that whenever the bridge amplifier I8 is unbalanced there is sufficient current flowing through the coils to maintain the pens or markers out of contact with the record discs 13, 90 and 9|. At the instant that une cut-off I2 reaches a position of balance the pens make a mark upon the charts. This thus indicates not only the focal length ofthe lens l but also the angular position in which a balance has been obtained, A manually operable switch |03 is provided for opening the marker circuit. This may be opened during the time the cut-off is being rotated to bring it into proper position with respect to the axis of astigmatism. .At that time the signal I8 indicates the presence or absence of a balanced condition of the light cells.

It sometimes happens that a lens to be tested has a very short focal length, in which case it would be necessaryto bring the cut-off I2 exceedingly close to the lens l. Under these conditions the margin of error would be increased.

To overcome this difficulty means is provided for moving the light slit 2 closer to the lens l, by a fixed amount. In this instance I have shown an arrangement wherebyrthe light slit may be moved to a position exactly half as far from the lens 4 from that illustrated in Figure 2.

As previously stated, and as illustrated in ligure 4, the slit is mounted to slide upon the rails 2|. The slit may occupy the position illustrated in Figures 2 and 3, or it may be pushed a fixed distance to the `right. When this is done it is desired to change the ratio of drive between the shaft 8| and the shaft 28. I'his is automatically accomplished in the following manner: clutch 11 is actuated by a bell crank ||0 pivoted at III and including a pin II2 riding in a slot II3 in an arm Ill carried by the sleeve 32. As the sleeve 32 is moved to the right the pin ||2 rides in the slot H3 until the sleeve reaches its extreme position at which point the arm Ill pushes the pin |I2 to actuate the clutch 11 to disengage the driving connection between the shaft 28 and the gear 15 and to establish a driving connection between this same shaft and the gear 16. The gear 16 drives the same stud shaft 8| as did the gear 15 but at twice the speed. Thus while the cut-off I2 is being adjusted with the light source half' as far from the lens the calibration is the same as though the light source were a distancefrom the lens illustrated in Figure 2 so that the calibrated scales 80 and 9| still read the focal length on that basis.

While in Figures 1 and 2 I have shown a testing system wherein the cut-off moves longitudinally towards the-lens, it is to be understood that the cut-off may be stationary and the lens 'full amounts of light.

moved towards the eut-on. Likewise, wnue the focusing system of Figure 1 is one wherein the incident light from the test' lens i`sconvergent, this also is notan indispensable part ofthe present invention. since the incident light mayconfsist oi' parallel rays, as illustrated in Figure--5.

Furthermore, the lens to be tested may consistv of a single lens or agroup of lenses assembled together. Such an arrangement is illustrated in Figure 5. Insofar as the parts of Figure are the same as those of Figure 1, similar reference numerals have been used. In this instance the light source I directs its light through a narrow longitudinal slit 2 in a plate I to-a lens |20 which directs the light rays into a `parallel beam I2| directed fto the lens system l', which system includes la series of lenses the focal point of which is to be ascertained. The lenses are mounted in a suitable holder which is mounted in the test apparatus so that the lens system I is movable in a direction parallel to the optical axis Il of the system. In this 'instance the cut-off I2 :is immovable in a direction `longitudinally of the optical axis I4. The light from the light source focuses at I5, which is the cross over point of the pencils of light from the lens system I', and then strikes the reflecting prism '6 as before, from which it -is reflected to the light cells 8 and I0 the outputs of which are directed to abalancing bridge type amplifier I8 as'heretofore. The system is balanced with the cut-off out of position, at which time both light cells receive the Thereafter the cut-off is positioned so that its knife edge reaches the optical axis I4 of the system and thus cuts off light from only one of the light cells. Thereafter Ithe lens system l is moved in a direction parallel to the optical axis I4 thus moving the focal point I5 of the `lens system closer and closer to 'the cut-off I2. This movement is of no effect on th\1ight cells until the cross over point I5 coincides with the knife edge of the cut-off I2, that is, until the lens is brought into focus on the edge of the cut-off. At this point, and only at this point, the balance of-the light cells is reestablished. If the lens 4 is moved too far to the right, so that the light is focused to the right of the cut-off I2, the balance of the outputs of the light cells 8-I0 is again destroyed for then the cut-olf masks out the light going to the cell I 0 rather than to the cell 8. Thus, only-at the cross over point is balance established.' 'I'he establishment of the balance thus indicates that the lens system is in a posi-tion such that it focuses the light I5 at the knife edge of the cuto. Theposition of the lens system may then be read upon a calibrated scale |25.

If the lens system is astigmatic, the axis of astigmatism must be ascertained as before. The cut-off is positioned so that its knife edge is at right angles to the apex of the prism 6, and then the lens system I' isv rotated about the axis I4 until a position is found where the light cells are balanced. Then the cut-oil.' is positioned with its knife edge parallel to -the apex of theprism and the lens system is moved along the axis I4 until a position of balance is again.reachedas previously explained.

. Figure 6 illustrates, diagrammatically,v a physical embodiment of thesystem of Figure 5. In this system the means for moving the lens system 4' longitudinally of the optical axis Il comprises a lens holder |28 in which the lens system 4 is mounted, which lens holder is in turn rotatably mounted in a holder 22' -through which a lead screw threads. Upon rotation of 'the lead screw -26' the holder |26 and the lens system 4' is advanced rectilinearly along the optical axis I4. The lead screw 26' may be rotated `in any desired manner, as by a worm wheel keyed to the lead screw and rotated by a worm I 3| on a shaft |32, which shaft is manually rotated.

Means is provided for rotating the lens system 4' about the optical axis I4 as a center. This means comprises a spur gear |34 which is a part of the holder |26 and which includes a cylindrical portion that is rotatable in the holder 22'. The spur gear |34 meshes with a'rota/table pinion shaft |33 that may be rotated in any desired manner as, for instance, by a worm wheel |36 keyed thereto, which is turned by a'worm |40 on a hand operated shaft |4I. Means, similar to those illustrated in Figure 2, may be provided for reading the angular position of the lens system 4 as well as the accurate electro-magnetic means of Figure 2 for indicating the rectilinear position of the lens when balance is obtained.

In Figure 6 I have shown no means for rotating the reflecting prism 6 with its. associated light cells, nor means for rotating the cut-off I2 or the slit plate 3, about the optical axis I4. This may, optionally, be provided, which structure would be the same as is illustrated in Figure 2. The illustration of this structure has been omitted for the sake of simplicity,I since that is already illustrated in Figure 2.

In 'Figure 7 I have illustrated a system wherein a lens or lens system may be tested to determine the focusing characteristics thereof under lights of dierent wave lengths. In this instance I have shown a testing system such as illustrated in Figure 5 (although it is to be understood that the system may be one such as illustrated in Figure l), wherein monochromatic light which passes through the slit 2 is obtained from a light source I and a dispersing prism |50. The light source and the prism are mounted on a stand I5I so that the light source together with the prism may be rotated. Light from the source I strikes the prism and is broken up into a band |52 ranging from the violet to the red or, if the source emits invisible light also, the range will be from the ultraviolet to the infrared. The plate 3 cuts off all of 'the light except light of the particular f wave length which passes through the slit 2. By rotating the prism |50 it is possible to bring any one of the colors of the spectrum from the prism opposite the slit 2 whereby the testing system tests the focusing characteristics of the lens system 4 for light or actinic rays of that particular wave length. Any other means for obtaining monochromatic light may be used.

In the previous descriptions I have described apparatus for ascertaining focal characteristics of lenses of the type which can be placed into the apparatus, and which focus light passing s through the lens. 'I'he principles of the present invention are also applicable to reflecting lenses and to lenses which cannot be placed into the apparatus as, for instance, the lens or fundus of the human eye. An apparatus for testing the focal characteristics and defects in the human eye is illustrated in Figure 8. The apparatus of' Figure 8 enables the testing of the human eye to ascertain not only the focusing characteristics of the eye but also to ascertain the astigmatism, if any, and the meridian or axis of astigmatism. A description will first be given of the construction of the apparatus of Figure 8 with a view of ascertaining the focusing characteristics of an eye being tested. The light source is indicated at I and directs its light through the slit 2 in the opaque plate 3, thence through a lens |20 from which parallel rays go to a transparent mirror |15. 'I'he mirror may be a plain-glass plate the opposite surfaces of which are parallel to one another and arranged in a plane at to the incident light. 'I'he mirror reflects an image of the slit 2 onto a mirror |16 which is parallel to the mirror |16 so that it directs the image of the slit along the path indicated, to the human eye |16 being tested. The image is focused on the fundus of the eye and reflected back to the mirror |18, thence to and through the mirror |16 to a focusing lens |60 of known focal length, from which the light passes to the reflecting prism 6 that directs the light to the light cells 6 and I6 the outputs of which extend to the bridge amplifier I8, in the manner previously explained. The circuits from the light cells are balanced against one anotherso that when equal quantities of light strike the two cells the electrical effects produced by the two cells counter-balance one another and the net output of the amplifier is zero. 'Ihe cut-ofi I2 is mounted in a ring 24', similar to the ring 24 of Figure 2, which In Aturn is rotatably mounted in a holder 22a that rides upon the tracks 2| similar to those illustrated In Figure 2. The ring 24' is moved longitudinally by moving the holder 22a by means of a lead screw 25 through a worm wheel 26 actuated by a worm 21 on a shaft 23 which is turned manually to turn the lead screw, all in the same manner as was heretofore yexplained in connection with Figure 2.

In Figure 8 I have shown the cut-off I2 at the cross over point I5 of the light rays, that is, at the point where the light rays reflected from the fundus of the eye are focused. At this point the cut-off I2 cuts off none of the light from the lens |60 to the prism 6 whereby a balanced condition of the amplier circuit is maintained. If the cut-ofi I2 is moved to the right or to the left from the cross over point I5 it' will cut oil' exactly half of the beam of light, whether the movement of the cut-olf is in one direction or the other and regardless of the amount of move'- ment. Thus the condition of balance of the light cells is disturbed. It is thus apparent that the balanced condition is obtained only at the cross over point I5. This is a very critical position so that even very slight deviations ofthe cut-on I2 from this position results in an unbalanced condition.

Means is provided for selectively rotating the slit 2 or the cut-off I2 or the prism -6. This means comprises a hand rotatable shaft 40 driving a worm 4I which drives a worm wheel 42 that turns a shaft 44'. The shaft 44' may be connected to turn the slit plate 3 by a clutch 46 which is actuated by a crank on a manually operated stud shaft 48. The clutch 46 connects the shaft 44' to a pinion 54 in mesh with a spur gear that turns the plate 3. 'I'he shaft 44' is also adapted to be connected by the clutch 45 actuated by the hand turned stud shaft 41 to connect the shaft 44 to the pinion shaft 50 which is in mesh with the spur gear 5I on the cut-off carrying ring 24 to turn the cut-off. Likewise the clutch 59 actuated by the crankon thestud shaft 60 is provided for connecting the shaft 44' to the gear 62 that is in mesh with the gear 63 that rotates the casing 35 in which is mounted the prism 6, light cells I-III and amplifier II, all ina manner similar to the'means used for producing the rotation of the corresponding parts in the structure of Figure 2. It is important to note that the cut-olf I2 and prism rotate about the optical axis I4 as a center and that the slit I rotates about the corresponding optical axis Il' as 'a center. Y a

Means is provided for ascertaining and recording the linear and angular positions of the cutoff at the time balance is obtained. For ascertaining the linear position of the cut-oi! the lead screw 2l is provided with a worm III that drives a worm wheel |90 on a stud shaft |81. 'Ihe stud shaft carries also a miter gear which is in mesh with a miter gear |90 that drives a shaft |9| which in kturn carries a spiral gear |92 in mesh with a spiral gear Ill that rotates a stud shaft |94 that carries a calibrated disc Ill. 'I'he gear ratio is such that the disc |95 makes one complete revolution for an amount of turning of the lead screw 2l necessary to move the cut-off |2 from one extreme position to the other extreme position.

The means for recording the angular position of the cut-olf comprises a worm200 on the pinion shaft 50, which worm drives a worm wheel 20| that'drives a stud shaft 202 that carries a miter gear 203 driving a miter gear 204 which turns a shaft 200 that is coupled with a stud shaft 206 bya pair of miter gears 201-208. The shaft 200 carries a calibrated disc 2|||.V 'I'he gearing ar.- rangement is such that. the disc 2|0 makes one revolution for a number of turns of the pinion shaft 50 necessary to turn the cut-oi! |2 through 360, although the gearing may be steppedyup so that the disc 2|0 makes one complete turn for one half turn of the cut-01T.

Electromagnetically controlled recording markers |00 and |0| are provided for making a visual impression upon the calibrated discs |85 and 2|.. These markers are maintained out of position with the calibrated discs or charts isb-2in ;until a balanced condition is obtained in the amplifier, at which time no current flows through the electro-magnetic coils, and the markers make their marks 'on the chart. This happens only when the cut-oilr I2 is at the focal point i5. 'Ihe charts thus indicate the position of the cut-oil! when balanced conditions prevail.

In order to permit visual examination of the d e patlents eye while the focusing characteristics 9 matic error, and the edge of the cut-o isnot in themeridian of astigmatism, it is not possible to obtain the focal length because when the cutoff is at an angle to the meridian of astigmatism the cells B-ll are unbalanced in every position of the cut-off on the optical axis. It is therefore necessary to ascertain the meridian of astigmatism. To accomplish this the knife edge of Vthe cut-on is `positioned at right angles to the apex of the prism and then the two are rotated together as a unit about the optical axis Il. The light cells remain unbalanced` until the knife edge of the cut-off reaches a position paralthe cells become balanced. The knife-edge is then moved through 90 (or the prism l is moved through 90) to bring the prism edge and the Vknife edge both into parallelism with one of the meridians of astigmatism. Thereafter the -cut' off isshifted on the axis i4 to ascertain the focal point in that meridian. The focal point in a meridian at right angles to that meridian'may then be ascertained by rotating the prism and the cut-oi! together through exactly 90' and then shifting the cut-olf 'along the optical axis I4 until a balance of the light cells is obtained.

In each of the embodiments of the present invention above described results are obtained either by moving the cut-off towards and away Y from the lens or by moving the lens towards and away from the cut-off. It is possible to obtain similar results even though the cut-oi! and the test lens are immovable axially' withrespect `to one another. cally in Figure 9. In this `ligure parts similar to those of Figure 1 have been given the same reference numerals. The test lens l is at a fixed distance from the slit 2. The cut-ofi', I2 is located between the lens 4 and the slit 2 and isl not movable along the optical axis, although it may be rotatable as in the embodiments previously illustrated. In this instance the cut-off I2 cuts oft light on one side of the optical axis Il "of the lens so that an unbalanced condition is obtained at the prism. When the prism C is in the position illustrated in Figure 9 no light vis received by the iight cell The prism is movable along the optical axis I4. left from the position illustrated in Figure 9, neither of the light cells 9-II is affected quantitatively by the movement until the prism crosses the cross over point i5. When the prism is to the left of the cross over point Il the light cell I0, which previously was receiving no light, immediately receives the full quantity of light,

and the light cell 8, which previously received the full quantity of light, now receives nok light. The balance is again disturbed but in the opposite direction. When the apex of the prism 8 is exactly at the point i5, both light cells receive equal quantities of light. The reestablishment of the balanced conditions thus indicates that theV frared rays, can be used so that dilation of the t pupil by the eye is avoided. In testing other lenses one can use infrared rays yor ultraviolet rays as may be expedient.

The instrument of the present invention may be used to measure the index of refraction of optical glass. The measurement of the kindex of refraction of a melt" of optical glass, at the present time, generally requires the formation of an optical at for this purpose. This is an expensive operaton. It is possible to measure the index of refraction of a melt of Voptical glass by i measuring the focal length of a lens made from lel to an axis of astigmatism. at which position that glass, which lens is of a known thickness and known radius of curvature. The focal length of a lens is determined by three factors.

namely, thickness of the lens, 'the radius of i curvature thereof, and the index of refraction of the glass. If a lens is made of a known or 'I'his is illustrated diagrammati- As the prism moves to the w measurableradius of curvature and the thickness thereof is measured and then the focal length y the test to determine the meridian of astigmatism is independent of the test to determine the focal length. As a result the rotation of the apparatus to determine the meridian of astigmatism is effective regardless of whether or not the apparatus is at a focal or cross-over point of the lens. In certain systems heretofore used for determining the meridian of astigmatism and the focal length of a lens it was necessary to shift the apparatus until a position is reached corresponding to the focal length and the meridian of astigmatism. This means if the meridian of astigmatism is reached when the focal length has not been reached, or vice versa, no indication will be given from which one of the two unknown factors can be ascertained. Since the two unknown factors, namely, the meridian of astigmatism and the focal length, are independent of one another, it isexceedingly difllcult to strike both the meridian of astigmatism and the focal length at the same time unless one knows when one of the unknown factors has been ascertained so that the search can thereafter be restricted to a search for the other factor. This is possible in accordance with the present invention. It is also apparent, from the above description, that the change from a condition of balance to a condition of unbalance, in the search for the meridian of astigmatism, and the change from a condition of balance in the search for ,the focal length is a sharp change that takes place critically as the apparatus is moved past the critical points. Thus it is not necessary for the operator to judge the position of the apex of a smooth curve or to judge the position of maximum indication of a signal of gradually varying intensity.

While in the present description I have spoken of rotating the apparatus with respect to the light beam, or rotating the lens to rotate the light beam, it is within the purview of the present invention to use other means for rotating the light beam. For instance, a suitable prism or prisms may be arranged in the light beam, which prisms may be rotated to rotate the light beam from the lens 4 with respect to the prism 6 or with respectto the cut-off I2.

In Figure 1 the cutofl I2 is a control member fcr controlling the illumination or lack of illumination upon the surfaces I and 9 of the prism 6. In Figure 5 the lens 4 is itself the control member for controlling the luminosity upon the different faces of the prism. In the modification illustrated in Figure 9 the prism itself is a control member for controlling the luminosity that is reflected from the surfaces 1 and 9 thereof to the light cells. In each instance the control member is moved to a critical position to bring the control member and the cross-over point I5 of light rays which are converged by the test lens into relationship determined by the focal length of the lens. Also, the control' member may comprise the plate 3 in which event the cut-off I2 is not movable along the optical axis I4. Since the focal length of the lens determines the relative relationship of the cross-over point I5 with respect to the distance between the lens and the light slit 2, it is apparent that if the light slit is moved towards or away from the lens 4 the cross-over point I5 is shifted. If now the lens 4 is at a fixed distance from the cut-oil I2 and the light slit 2 is moved toward the lens until the cross-over point I5 coincides with the knife edge of the cut-off I2, the distance between the lens 4 and the light source 2 will be a function of the focal length of thev lens. The focal length may thus be calibrated or charted in terms of the distance between the light slit 2 and the lens 4.

In compliance with the requirements of the 4 patent statutes I have here shown and described a few preferred embodiments of my invention. It is, however, to be understood that the invention is not limited to the precise constructions here shown, the same being merely illustrative of the principles of the invention. What I consider new and desire to secure by Letters Patent is:

1. Means for determining focusing characteristics of a lens which comprises a pair of light responsive cells receiving diametrlcally opposedV segments of light focused by the lens, means for balancing the effects of the cells. a light control member in the beam of light focused by the lens and extending up to the axis of the optical system of the lens, said lens and said member being movable with respect to one another along the optical axis of the lens to bring the cross-over point of light from the lens and said member into a predetermined relative space relationship, said member controlling the passage of light to both cells.

2. Means for determining the meridian of an axis of astigmatism of a lens, which comprises means for causing a beam of light to be diverted by the lens, a pair of light lresponsive cells for receiving diametrically opposed segments of the beam of light, means for balancing the effects of the cells, a light control member intercepting the beam of light on one side only of the optical axis of the lens, and means for relatively rotating the light cells and the control member and the beam of light from the lens with respect to one another about the optical axis of the beam of light as a center to change the condition of balance of the cells.

3. Means for determining characteristics of a lens, which comprises means for directing a beam of light to the lens, a pair of light responsive cells for receiving diametrically opposed segments of the beam of light from the lens, means for balancing the effects of the cells, a light control member intercepting the beam of light on one side only of the optical axis of the lens, means for relatively rotating the control member and the beam of light with respect to one another about the optical axis of the lens, said control member and lens being relatively movable along the optical axis of the lens to bring the crossover point of light from the lens and said member together, said member controlling the passage of light to both cells and establishing a balanced condition of the cells when the cross-over point of light from the lens is adjacent the member.

4. Apparatus for testing characteristics of a focusing optical device comprising means for dlrecting a beam of light to the device, and thence towards a cross-over point, a light responsive cell positioned to receive only half of a beam of light which is focused by the device, and a mask in the path of the beam and extending up to the .optical axis of light focused by the de-` vice and cutting half of the beam, said mask and side of the cross-over point it cuts oi! the half of the light beam which affects the light cell and when the mask is on the other side of the crossover point it cuts of! the half of the'light beam which is ineffective on the light cell.

5,'Apparatus .for testing characteristics of a focusing optical system, said apparatus comprising means for directing a beam of light to the device, and thence towards a 'cross-'over point, a

` pair of light responsive cells ofA which one receives' light predominantly from one half of the system and the other receives light predominantly from the other half of the system, means for balancing the effects of the two cells, a mask in the system, said mask extending up to the :1.

optical axis of light focused by the device and obstructing light from one half of the system and obstructing no light from the other half of the system when the mask is in a position off of the cross-over point of light from the system and means for moving the mask and the beamrof focused light with respect to one another along\ the axis of the beam to bring the mask to a position on the cross-'over point of light of the system whereby the effects of the cells balance one another thus indicating the attainment of a position of the cross-over point with respect to the mask in the system.

6. In an 'optical system, means for determining focusing characteristics of la lens which comprises a pair of light cells, means including the lens to be testedA for directing light towards a cross-over vpoint and to both cells, .means for balancing the effects of the `two cells, said optical system including a control member, said member and said lens beingmovable with respect y to one another along the optical axis of the system, said control member permitting the pas-V sage of equivalent amounts of light from the lens to both cells only when the cross-over point of the light from the lens is at a predetermined position with respect to the member and altering the relative amounts of light to said cells in other operative positions of the member, whereby the establishment ofrbalance of the cells connotes that the cross-over point of light from the lens is locatedat said predetermined position with respect to the member.

7. The method of testing a lens which comprises directing light fromthe lens through a cross-over point to a light sensitive electric cell, positioning a cut-off in the path of the beam of the piane ci the fm1 axis or the lens Athrough a cross-over point towards two balanced iight fromv the lens to the edge oi' the mask to' direct light past the mask to both cells Vto reestablish the balance of the effects of the light cells, and by the reestablishment of the balance ascertaining that the cross-'over point and the edge of the mask have been broughttogether.

9. The method of testing a lens which com Prises directing light from two halves of the lens through a cross-over point to different ones of Y two optical' systems, placing a cut-oi! inthe pathl of the Alight beam from the lens to cut on one of the above halves ofthe light from the lens, moving the lens and the cut-oi! with respect to'each other along the optical axisl of the lens until the cut-ofi` is at the cross-over point 'of light from the lens whereby at thatl pointthe effect of theeut-os is the same on both systems,

and ascertaining the position of the4 cross-over point by determining the position where the light eiiects of the two optical systems, are the same. 10. The method of determiningl a meridian of astigmatism and the focal length of a lens in,

that meridian, which comprises positioning a cutoi member' anda straight edge'light dividing member so that the ledge of said cut oi! member is at 90` to said straight edge in a beam otlight from the lena-relatively rotating the light beam iight that passes through the .lens with the edge of the cut-ofi terminating at the optical axis of the light beam and by said cut-off cutting off all of the beam that is on one side of the focal plane of' the lens while permitting some of the light at the opposite side of the focal plane to pass to the light cell, and moving the lens and the cut-ofi.' with respect to one another along the optical axis of the light beam until the positions of light and darkness on opposite sides loi the focal plane are reversed and ascertaining the position oi reversal of light and darkness by ascertaining the position at which there is an abrupt change in the electric output of the cell 8. In an optical system the method of determining focal characteristics of a lens which comprises directing a beam of light towards the lens and directing the light from opposite sides of and the members with respect to one another while maintaining the same angular relation be tween the members, directing the light from two t halves of the light dividing member to al balanced photo-electric system, continuing the rotation until there is a change as between ba, unbalance of the system, then bringing the edges of two members into a condition of parallelism to one another, then producing relative motion between at least one of the members and the cross- 'over point of the beam of light from the lens in a'direction along the optical axis of the beam, a

continuing the last mentioned motion until there is again a change in the condition of balance or unbalance of the photo-electric system, and ascertaining the angular and linear positions aty which'the two changes in condition of balance l' or unbalance of the system took place.

11. Apparatus i'or testing a focusing optical device, coniprising a lightcell, means for causing the device to direct a beam of light through a cross-over point and then tothe cell, means for dividing a beam of light into twoyparts by a line said light dividing means relatively to one another parallel to the optical axis ofthe beam` 1 to bring the cross-over point of light focused by the device rst on one side o! the light dividingl means and then on the other side thereof, whereby there`is a sharp change in the light reaching the light cell when the light dividing means is at the cross-over' point, and electric means controlled by the light cell responsive to such change.

12. Apparatus for testing a focusing optical device, comprising Va light cell, means ioricaus-V ing the device to direct a beam of light through a cross-over point and then to the cell, means for dividing a beam of light into two parts by a line of division which passes through the optical axis ofthe beam and directing only one of said divided parts of the beam to the light cell, means for relatively rotating the beam of light and said last named means with respect to one another about-the optical axis of the beam, means for moving the cross-over point of the beam and said light' dividing means relatively to one another parallel to the optical axis of the beam to bring the cross-over point" off light focused by the device ilrst on one side of the light dividing means and then on the other side thereof, whereby there is a sharp change in the light reaching the -light cell when the light dividing means is at the crossover point, and electric means controlled by the light cell responsive to such change.

13. Apparatus for testing a focusing optical device. comprising a light cell, means fox-causing the device to direct a beam of light through a cross-over point and then toward the cell, a member dividing the beam of light into two parts by a line of division which passes through the,

optical axis of the beam and directing only one of said divided parts of. the beam to the light cell, a v

cut-oir member intercepting apart of said beam of light between the light source and the said dividing member and having its edge substantially along the optical axis of the beam and parallel to thesaid line of division, means for moving the cross-over point of the beam and one of said two members relatively to one another parallel to the optical axis of the beam to bring the cross-over point of light focused by the de vice first on one side of the one member and then on the other s ide thereof, whereby there is a sharp change in the light reaching the light cell when the cross-over point and the said one member are coincident, and electric means controlled by the light cellresponsive to such change.

14. Apparatus for testing a focusing optical device, comprising two light responsive cells, means for causing the device to direct a beam of light through a cross-over point andthen toward the cells, means for dividing a beam of light into two parts by a line of division which passes through the optical axis of the beam and directing one of said divided parts of the beam to one light cell and the other divided part of the beam to the other light cell, means for moving the cross-over point of the beam and the means for dividing the beam relatively to one another parallel to the optical axis of the beam to bring the cross-over point of light focused by the device rst on one side of the means for dividing the beam -and then on the other side thereof, said second named means` including a cut-oil intercepting said beam of light between the light source and the said cells and having its edge substantially along the optical axis of the beam and parallel to the said line of division whereby the cut-off intercepts one of said halves Aof the beam, and there is a change in the light reaching the respective light cells when the means for dividing the beam passes the crossover point, and electric means controlled by the` light cells responsive to such change.

l5'. Apparatus for testing a focusing optical device comprising two light responsive cells, means for causing the device to direct a beam of light through a cross-over point and then toward the cells, means for dividing a beam of light into two part-s bya line of division which passes through the optical axis of the beam and directing one of said divided parts of the beam to one light cell and the other divided part of the '9 beam to the other ngnt'ceu, a cut-on intercepting said beam `of-light between the light source and the said dividing means and having its edge substantially along the optical axis of the beam and parallel to the said line of division whereby the cut-oi! intercepts one of said halves of the` the device first on one side of the cut-off'and then on the other side thereof, whereby the .cut-oir intercepts difi'erent portions of the beam when positioned on different sides of the crom- `over point oi?V light focused by the device and there is a change in the light reaching the respective light cells when the cut-oi! is at the cross-over point, and electric means controlled by the light cells responsive to such change.

16. Apparatus for testing a focusing opticaly device comprising two light responsive' cells, means for causing the device to direct a beam of` light through a cross-over point and then toward the cells, means for dividinga beam of light into two parts by a linev of division which passes through the optical axis of the beam and directing one of said divided parts of the beam to oneilight cell and the other divided part of the beam to lthe other light cell, a cut-oir intercepting said beam of. light between the light source and the said dividing means and having its edge substantially along the optical axis o1 the beam, means for rotating the means for dividing the beam and the out-oil relatively to one another about the optical axis of the beam of light vand means for producing relative rotationv between the beam of light as one unit and the cutoif and the second named means as another unit about the axis of the beam of light, means for moving the cross-over point of the'beam and the cut-ofi relatively to one another parallel to the optical axis of the beam to bring the crossover point of light focused by the'device first on one side of the cut-oil' and then on the other side thereof, whereby the cut-oir intercepts diierent portions of the beam when positioned on different sides-of the cross-over point of light focused by the device and there is a change 'in the light reaching the respective light cells when the cut-oil' is at the cross-over point, and electric means controlled by the light cells responsive to 'such change.

1'7. Apparatus for testing a focusing optical device, comprising a light sensitive cell, means for causing the device to be tested to direct a, beam of light through a cross-over point and direct half of the light beam onto the cell, light intercepting means, means for holding the intercepting means in the path of the light beam with the edge of the intercepting means terminating substantially at the optical axis of the light beam, means for moving the intercepting means and the cross-over point of the light beam with respect to one another in a direction substantially along the optical axis of the light beam while maintaining the edge of the intercepting means substantially along the optical axis of the light beam, whereby the intercepting means intercepts substantially one half of the light beam when the intercepting means is on one side of the cross-over point of the light beam and intercepts the other half of the light beam when the intercepting means is on the other side of said cross-over point and there is an abrupt change in the illumination striking the light sensitive cell as the interceptingmeans and the cross'- over point of 'the light Vbeam pass one another, and an electro-responsive device controlled `by the light sensitive cell .for indicating :that the interceptingmeans and the cross-over pointare in the position at which the abrupt changevin v the cut-off and the edge of therrefiector both terminating substantially at the optical axis of the light beam, means for relatively rotating the cut-off with respect to the reflector about the optical axis of the beam to bring them into different 'angular relationships, means for relatively rotating the intercepting means as a unit and the beam of light'with respect to one another about the optical axis of the beam, means for moving the cut-off and the cross-over point of the light beam with respect to one another in a direction substantially along the optical axis of the light beam while maintaining the edge of the cut-oil substantially along the optical axis of the light beam, whereby the cut-oi! intercepts substantially one half of the light beam when the cut-oi! is on one side of the cross-over point of the light beam and intercepts the other half of the light beam when the cut-of! is on the other side of said cross-over point and there is a change in the illumination striking the light sensitive cell as the cut-off and the cross-over point of the light beam pass one another, and an electro-responsive device controlled by the light sensitive cell for indicating that the intercepting means and the cross-over point are in the position at which the said change in illumination occurs.

19. Means for testing light focusing apparatus comprising means forming an optical system including the light focusing apparatus to be tested, said system including means for directing a light to the apparatus and from the apparatus to a cross-over point at a location determined by the focusing characteristics of the apparatus, light responsive means, light control means directing light from only one side of the optical r.

axis of the system to the light responsive means, means for moving the cross-over point and the light control means with respect to one another along the optical axis of the system to change the amountV of light directed to the light responsive means as the control means and the crossover point come into a predetermined position with respect to one another, and electric actuated means controlled by the light responsive apparatus in response to the change in the amount of light received thereby.-

20. Testing equipment for determining a meridian of astigmatism of light focusing optical apparatus, said equipment comprising means forming an optical system including the light focusing apparatus whose meridian of astigmatism is to be determined, said system including -means for directing light to the apparatus and then from the apparatus so that the cross section of the light' beam from the apparatus is altered by theapparatus in accordance with the astigmatism thereof, means dividing. oif a quadrant of the -light beam by lines of division at right anglesto one another and both lines extending. through the optical axis of the system,

means for rotating saiddividing means and the y light beam fromthe apparatus with Wt to Y one another about the optical axis of the system wmie 4mimaimm; the unes of division at right n angles to 'one another', light `sensitive means :re-

ceiving light from said quadrant and means controlled by the light sensitive means responsive to` changes in light receivedthereby in diiferent then from theapparatus to a cross-over pointy at a location determined by. the focusing characteristics of the apparatus, placing a control memberin the path of the light and by said member directing light from onlyl one side of-the optical axis of the system to a light responsive Amember, moving the position of the light control member and the cross-over point of the light with respect to one another along the optical axis of the system and producing a change in the light directed to the light responsive member as the cross-over point and the control member come into one predeterminedposition with respect to one another, and bythe change in the light responsive vmember determining the position of the cross-over point.

22. The'method of determining the meridian of astigmatism of a focusing optical apparatus which comprises producing a beamv of lightfocused by the apparatus, dividing oi! a quadrant of said beam by two 'lines of division at right angles to one another and each extending lup to the optical axis of the beam of light, altering the light of the quadrant by rotating the lines of division and the beam of light with respect to each other about the axis of the beam while maintaining the lines of division at right angles to one another, directing the light of the changing quadrant, to a light cell, and ascertaining the angle between the lines of division andthe light e source positioned to direct a beam of lightk through said lens and converge the same through a cross-over point, means associated with said beam to'detect relativelight variations in diametrically opposed segments of said beam, meansv movable to and from the cross-over point of said beam along the optical axis of the beam tol vary the light intensity of one of said segments, said last means being ineffective at said cross-over point to vary the relative light intensity of said segments, and means controlled by said second named means in response to the change in the amount of light received thereby to indicate the location of the cross-over point.

24. In an apparatus for determining the focal length of-a lens, the combination with a lens support and means for directing a beam of light on,

a fixed axis through a lens on said support and to converge the beam towards a cross-over point, of means to detect relative light variations in diametrically opposed segments of said converging beam beyond the cross-over point of said lens. means movable to and from the cross-overv means controlled by said second-named means in response to the change in the amount of light received thereby to indicate the location ofthe cross-over point.

25. The method of determining the focal point 10 of a lens comprising directing a beam of light through said lens and thence through a crossover point, varying the light intensity of one of two diametrically opposed segments of said beam by moving a shield longitudinally Valong the optical axis and at least to the cross-over point, -and detecting relative variations of the light intensity of the two segments by means of a balanced circuit meter having light sensitive means in the path of each segment.

CHARLES J. GLASSER. 

